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Hybrid Fireclay Aerocrete Oven

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  • Hybrid Fireclay Aerocrete Oven

    I'm new to this fantastic forum. Thanks.
    My wife is adamant that our oven will feature a clay dome as the main cooking area thermal mass. She doesn't want cement in her oven apparently.
    I'm not so keen on the aesthetics of completely rustic cob built ovens and their problems with the bad weather we have here.

    We are building in the back yard on a raised area partly beneath a canopy.
    That means our cooking space is very near to the door to the kitchen and is undercover, which is great.
    I think I can get away without building a further base as we are going onto stone and concrete flags on mortar and hardcore.

    Because of the location we have a canopy to avoid with the flue so will need to duct from the ideal front position to the middle the dome where it will clear the canopy.
    An easy option would be to build a cob oven and enclose it in a 'kennel' to weatherproof it and hide any ugly flue routing we might have to do, with loose fill insulation.
    Unfortunately we don't quite have the space at the back of the oven for an enclosure without interfering with existing hard landscaping.

    I am proposing hybrid design, using a basic 2:1 sand:fireclay mix 'cast' over a sandcastle dome form to create a 55mm thick oven inner.
    This will be sitting on standard 64mm firebricks on 50mm ceramic board insulation.
    The clay thermal mass layer would be dried completely before adding 25-50mm ceramic blanket insulation and a layer of cement + aerated clay 'aeorcrete' for more insulation and protection.

    The outer layer would be 100mm thick and incorporate ducting the flu from the ideal position up to the top of the dome and a twin wall chimney.
    If I am able to get this layer tidy and waterproof I would not add any render. If needs be the whole thing will also be rendered.
    I am hopeful I can make the aerocrete layer so that it could be removed if we had cast replacement clay dome.

    As I understand it because of the shrinkage of clay ovens and the fact that they aren't actually fired at ceramic temperatures it is normal to add any outer layers before the inner has dried and let thewole lot slowly dry out together. I am hoping that by 'uncoupling' the clay layer from the concrete outer with the ceramic blanket I can wait until the clay has done all of it's shrinking before moving on to create the outer layer, thereby having more control of dimensions.

    Apologies for the long first post. I have attached a sketch of the design and welcome comments and advice.
    Last edited by blink; 03-04-2021, 04:15 AM.

  • #2
    Welcome to the forum! I can't offer any advice regarding clay ovens but your design is similar to mine, although mine is made from brick. I have found that the flue over the dome or "squirrel tail" flue works well.
    My 42" build: https://community.fornobravo.com/for...ld-new-zealand
    My oven drawings: My oven drawings - Forno Bravo Forum: The Wood-Fired Oven Community

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    • #3
      Thanks. I’m sorry that I hadn’t realised it was a thing! At least I know to search for ‘squirrel’.

      I guess with brick built its straightforward to construct the tail to the top flue.

      I still might rethink the whole thing about just doing a clay dome in a kennel and use ss flue liner to duct to the flue and bury it in loose insulation.

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      • #4
        Clay has pretty high shrinkage rate even if your mix is 2 parts sand 1 part clay. If you build over a sand form when the cob mix dries it will shrink and cracking is likely. It is best to remove the sand as soon as the clay become what potters call leather hard. At this stage the shrinkage is not too substantial but the structure will be self supporting.
        Regarding your aerocrete I presume you are looking at lightweight expanded clay aggregate LECA. You may want to consider vermicrete or perlite as an alternative which are both cheaper (at least for me) then LECA. It is imperative that you understand the cement content used to bind any lightweight aggregate reduces insulating value enormously. The attached table explains it.

        Click image for larger version

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        Kindled with zeal and fired with passion.

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        • #5
          Thanks. Yes, I'm aware of clay shrinkage general terms, which can be from 4 to 20% so I will do some tests with my mix. I have also read it's important to get the sand mould out as soon as possible for this reason. How long it will take a 900mm diameter lump of 55mm thick clay to firm up enough is still an unknown for me, seems like it could be 6h 12h, twice that or more. I'm also worried about it sticking at the bottom and not being able to shrink in diameter at this point, leading to more cracking. And making the mould the correct dimensions so that I end up with a dome that is the correct size for the firebrick base. I suspect I am going to have to make my brick base a bit wider than necessary, consuming extra expensive firebricks, to be sure it all ends up as required.

          When I spoke about allowing it to dry completely, that was just in terms of adding the next layer.

          I can get 700litres of aerated clay for a little over a hundred GBP, with delivery on top. And I have looked into some of the issue with making cement mixes with this kind of material.

          As it turns out I may well be hijacking my own thread and reverting to the alternative plan of a simple clay dome (this part is the same in both designs) in an enclosure loose filled with created clay insulation. I have worked out how to keep my construction compact enough to fit in the space and I'm not sure the double dome option will give me enough support for the length of flue I need get far enough beyond the eaves of a canopy I need to clear.

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